The back-thinning of substrates, in particular wafers, is often necessary in the semiconductor industry and can be done mechanically and/or chemically. For back-thinning, the wafers are generally temporarily attached to a carrier system, by various methods of attachment. As carrier systems, films or wafers made of, for example, silicon, silicon alloys such as SiC, SiN, etc., ceramics, (glass-fiber-reinforced) plastics, graphite, sapphire, metals, glasses or composite materials are used. At the end of the back-thinning process and the post-processing, the back-thinned wafers are mounted on film holders, and then the carrier is removed.
Whenever a working of the substrate, that goes beyond the back-thinning, is necessary, rigid carrier systems, namely carrier substrates, are used. Examples of such working steps after back-thinning are: metallization, dry etching, wet etching, laser processing, lithography, oven processes, doping, etc.
In the case of a rigid carrier substrate, the product substrate that is to be worked is typically connected by an adhesive layer to the carrier substrate.
The carrier substrate is to impart adequate mechanical stability to the substrate that is to be worked in order to be able to be worked in related process steps or process devices. In the case of a temporary connection, target thicknesses are now: between 30 and 100 μm; in the future, thinner product substrates are targeted between 1 μm and 50 μm. In the case of a permanent connection, still thinner product substrates are possible, which are physically limited only by the requirements as regards the height of a transistor with connections. The minimal thicknesses of a product substrate are between 0.001 μm and 5 μm.
Some of the above-mentioned working steps require an exact positioning of the substrates or the carrier within the corresponding devices.
In this case, product substrates with a nominal 300 mm+/−200 μm, for example, are bonded to the carrier substrate with 301 mm+/−200 μm. This is done as a precautionary measure in order to adequately protect, and in particular to support, wafers in the edge areas that are to be back-thinned or that are back-thinned. Because of this measure, the carrier substrate is, however, unattached in the edge area in various working steps, in particular in sputter processes, galvanic deposition, and etching processes.
Because of the carrier substrates mentioned in the state of the art, several problems result. Deposition processes, etchings on the edge of the carrier substrate, etc., result in the carrier substrate edge being heavily contaminated.
After detaching from the product substrate, this contaminated edge area must be purified, at great expense in cost and labor. Often, the defective carrier substrate edge is the sole factor that limits the service life of the carrier substrate. The additional costs for an end product follow from, the costs of the carrier substrate, its recycling costs, and the number of reuse cycles. By this previously used process, a purification step of the carrier substrate is very costly. As a result, in many cases, the carrier substrate is not reused.
The more advantageous the carrier substrate, the less critical is a small number of reuse cycles; for example, at least ten reuse processes are desired for carrier substrate production costs of around 20.
The more costly the carrier substrate, the more important is its long service life (=large number of reuse cycles). For example, 1,000 reuse processes are desired for carrier substrate production costs of around 2,000.
Properties that can make a carrier substrate costly in the first production are, e.g.:                Starting material,        Precise geometry: low TTV (Total Thickness Variation), e.g., <1 μm necessary to be able to smooth and polish the product as precisely as possible to the desired thickness,        Pretreatments that make possible a subsequent detaching of the temporary bond.        
Because of these problems, very costly carrier substrates are frequently not used at all, even though they had useful properties for other process steps.
In the process steps cited below, very stringent requirements exist as regards the accuracy of the alignment of two wafers:                In the case of plasma working of back-thinned wafers on carrier substrates, an eccentricity produces an uneven discharge of the plasma. Discharges that are produced (breakdowns because of high electrical field density—arcing) can cause damage to products and plasma process chambers. Special advantages in plasma and sputter processes are achieved because of the possibility of using a carrier substrate that is the same as/smaller than the product substrate.        In the case of lithographic exposure on so-called scanners and steppers, inadequately adjusted bond pairs are not loaded with sufficient accuracy. The referencing (pre-alignment) of the bond pair is done based on the outside contour. The outside contour of a (much) larger carrier substrate, however, does not correspond to the position of the passmarks on the product substrate as long as the adjustment of the two outside contours is not precise, or the outside contour of the product substrate cannot be used. The passmarks are thus not in the “capture range” of the microscope, and a laborious search must be made for them. This leads to losses in time, production throughput, and productivity in these systems.        
An advantage of this invention is a device and a process for producing a substrate-carrier substrate combination or a substrate wherein a more exact and more efficient alignment and bringing substrates into contact with a carrier substrate is made possible.
This advantage is achieved with the features of claim 1. Advantageous further developments of the invention are indicated in the subclaims. Also, all combinations that consist of at least two of the features indicated in the specification, the claims and/or the figures fall within the scope of the invention. In the indicated ranges of values, values that lie within the above-mentioned limits are also to be disclosed as boundary values and can be claimed in any combination.
The invention is based on the idea of optimizing the process according to the invention by providing a substrate with a larger diameter d1 than the diameter d2 of the carrier substrate when contact is produced and providing a device with which the process is feasible. In this respect, an electronic detection (detection means) of outside contours of the substrates to be brought into contact and to be aligned, as well as processing (control means) of the detected outside contours in control signals for aligning (alignment means) the substrate, are conceivable according to the invention to achieve a more exact alignment. According to the invention, in addition, the alignment is preferably done continuously when substrates are moved on one another to bring them into contact. Moreover, it is conceivable to examine the alignment accuracy with the same detection means and optionally to perform a renewed alignment.
The diameters d1 and/or d2 are measured parallel to the contact side or the support surface, whereby the latter are to be regarded as mean diameter d1 and/or d2 (averaged along the peripheral contours of the substrate/carrier substrate). Relative to the cross-section of the substrate/carrier substrate and the respective cross-sectional contour, the mean diameters d1/d2 are measured on the respective maximum of the cross-sectional contour. Ideally, the substrate and the carrier substrate are exactly circular, so that the diameters d1/d2 do not deviate from one another on the periphery.
Substrates are defined as product or carrier substrates used in the semiconductor industry. The carrier substrate serves as an enhancement of the function substrate (product substrate) in the different working steps, during back-thinning of the function substrate. Suitable substrates, i.e., wafers, come either with smoothing (“flat”) or grooves (“notch”).
As an independent invention, a product (or a substrate-carrier substrate combination) is provided, which is comprised of a carrier substrate and a substrate, which have been aligned, brought into contact and prefixed with one another and/or bonded with the device according to the invention and/or the process according to the invention and are distinguished in that the diameter d2 of the carrier substrate is minimally smaller than the diameter d1 of the product substrate. According to the invention, it is thus ensured that the carrier substrate during the processing of the product substrate is not exposed to any contamination, fouling or unintentional treatment, etc., whatsoever and therefore can be reused more frequently.
Although the embodiment according to the invention is primarily suitable to align a carrier substrate that is smaller as far as the diameter d2 is concerned, relative to a substrate that is larger as far as the diameter d1 is concerned, a device according to the invention can also be used to align carrier substrates toward one another, where the carrier substrates are larger than or the same size as the substrates that are to be bonded.
According to a further development of the invention, it is advantageous, when the substrate is back-thinned after being brought into contact, that the diameter d1 is reduced by the shape of the cross-section of the substrate on its peripheral contour, in particular to d1<=d2. As a result, the simple further processing of the substrate-carrier substrate combination, in known and standardized units, is made possible.
In this case, it is of special advantage that the substrate has an annular shoulder that is produced in particular by providing an edge radius and/or by looping back the peripheral contour. The latter can be produced in a simple way and contributes to the further optimization of the production process according to the invention.
By means of an annular width dR of the shoulder being larger than or equal to the difference between d1 and d2, the diameter of the substrate can be reduced to the diameter d2 of the carrier wafer or smaller, so that in the further processing, an optimal support of the product substrate is ensured.
According to another advantageous embodiment of the invention, during back-thinning, a thickness D1 of the substrate is reduced up to or over the shoulder.
The device according to the invention is further developed in that the detection means are designed to detect the shape of the cross-section of the substrate on its peripheral contour in such a way that the back-thinning of the substrate can be controlled so that the diameter d1 is reduced, in particular to d1<=d2. By the detection of the shape of the cross-section, a profile of the cross-sectional contour viewed from the side, i.e., along the thickness D1 of the substrate, an exact control of the back-thinning process can be carried out.
By the detection means being rotatable relative to the substrate and/or relative to the carrier substrate by rotational means and/or adjustable parallel to the contact plane relative to the substrate and/or relative to the carrier substrate by an adjustment system in the X- and/or Y-direction, the alignment can be implemented efficiently and precisely.
According to an advantageous embodiment of the invention, that the detection means are attached to a carrier unit that is designed in particular to be annular in sections and that can be arranged at least in sections on the peripheral side with respect to the substrate and/or carrier substrate. Thus, an integration according to the invention of the detection means is possible in an efficient way.
Advantageously, the carrier unit is attached between the carrier substrate holding means and the substrate holding means, with contacting means attached in-between, preferably in the form of a Z-adjustment unit and/or with a base plate attached in-between. In this way, an efficient configuration of the invention is provided.
In this case, in further development of the invention, it is advantageous when the adjustment system is attached, directly, between the base plate and the carrier unit. Thus, a direct action on the carrier unit, together with the substrate holding means attached thereto, is conceivable.
According to another advantageous embodiment of the invention, the peripheral contour and the peripheral contour (both peripheral contours) are detectable, simultaneously, with the same, one or more, detection means, preferably microscopes. In this way, the number of costly detection means can be reduced without sacrificing speed.
The alignment of a substrate can also be done in a substrate stack as a carrier substrate. In this case, a substrate stack is defined as a number of already worked, for example back-thinned, substrates, which are bonded with one another permanently. According to the invention, this substrate stack, when it is thick enough, can serve as a carrier substrate.
The use of a mechanical alignment system (adjustment system and rotational means) is preferred both for the substrate and for the carrier substrate in connection with an optical distance-measuring system. In this connection, it is of special advantage when this alignment system is arranged integrated in a unit for bonding or prefixing the substrates.
The invention thus allows an exact, quick and economical alignment of two substrates (substrate and carrier substrate) with respect to one another, without having to refer to the alignment marks. According to the invention, carrier substrates can therefore dispense with alignment marks, so that the latter can be produced more advantageously.
In addition, because of this invention, repeated use of the carrier substrate is possible, without the latter having to be purified by labor-intensive and costly processes.
Moreover, the possibility arises of incorporating/integrating the device according to the invention in a bonder.
With this invention, the different diameters of the substrates at several points of the periphery (peripheral contours) are taken into consideration, and a more precise positioning is made possible, which factors are not possible in the case of other mechanical and/or optical positioning processes. At the same time, a faster alignment than in the case of (purely) mechanical alignment is made possible.
The process underlying the invention and the device according to the invention are able to make possible the necessary accuracies, in particular <50 μm, and in particular to achieve a higher rotational accuracy on the periphery, which is a weak point in the case of previous mechanical processes. Very precise grooves can be assigned on the periphery of the substrate (so-called “notches”) of the substrate and carrier substrate.
Advantageously, the grooves are located in such a position that they are detected by at least one detection means. Any detection means whose direction of measurement is parallel, or almost parallel, to the substrate normal can determine a rotational mis-orientation quickly by the position of the grooves. Similar considerations apply to substrates with flats or any other grooves or deviations from a predetermined ideal geometry of the substrate in question, which can be used for rotational alignment.
The process according to the invention is a dynamic, optical scanning process with software-controlled optimization of the detected/measured data.
In an advantageous embodiment of the invention, the carrier substrate and the function substrate in each case is attached to separate, mechanically movable holding devices (substrate holding means/carrier substrate holding means). In this connection, chucks with vacuum holding devices are provided. Also, however, other holding devices, such as adhesive materials, mechanical clamps, or electrostatic holding devices, can be provided. Also, instead of the carrier substrate, production substrates can also be provided. Also, already repeatedly bonded or back-thinned multi-layer substrates can be adjusted/aligned with this process.
Advantageously, one of the two substrates is attached on a fastening system (a carrier substrate holding means) that can move in the z-direction. The other substrate is attached on a rotatable chuck (a substrate holding means). The latter is fastened in a mechanical adjustment unit (a carrier unit), which can be adjusted in the x- and y-direction. On/in this mechanical adjustment unit, there are one or more optical scanning units (detection means), which detect(s) (scan(s)) the two substrates, simultaneously, in a narrow band range in the vertical direction. As a result, a gap profile is produced that simultaneously detects/measures the outside geometry (peripheral contours) of the two substrates. This gap profile produces the largest outside diameter of the respective substrate and the distance between the individual substrates and the measuring points or measuring sections.
These scanning units of the detection means advantageously rotate in the mechanical adjustment unit in—or parallel to—the contact plane in order to make possible a detection of several peripheral sections of the peripheral contours of the substrates. By the rotation of the scanning units, it is possible to measure the outside geometry of the two substrates and at the same time to determine the position of the two substrates with respect to one another. The rotation can describe a full circle or else only sectors/detection sections. For less precise adjustment requirements, rotation of the scanning units can be eliminated. It is also possible not to rotate the scanning units, but rather to rotate the two substrates in the case of stationary scanning units.
As an alternative, several scanning units can also be arranged in a stationary manner around the two substrates, in particular at least 3, in order to determine the position and the diameter of the two outside contours (or idealized circles that correspond in software). In this case, a rotation of substrates and/or scanning units can be omitted. A slight rotation of one substrate relative to the other substrate is in this case sufficient to align the notches or the flats in the rotational direction in the contact plane.
As an alternative, the scanning units can be arranged approximately at right angles to the substrate plane from above or below and can detect the edges of the peripheral contours of the substrates. In a special case, these are microscopes that provide an optical image of the two wafer edges for measurement and evaluation.
One or more of these microscopes can be arranged to be movable according to the invention (rotating around fixed/standing substrates) or stationary (with rotating substrates).
In another special case, at least three microscopes are arranged in a stationary manner on the periphery above and/or below the substrates. Both peripheral edges are visible via the microscopes (optionally by refocusing because of the Z-distance (crosswise to the X- and Y-direction or contact plane), which could exceed the depth of focus. The substrate with the larger diameter, which could obscure the view on the peripheral edge of the smaller substrate, can be moved a certain distance by the alignment means and can thus be made visible and positionable. The image information of the two peripheral edges is converted into a piece of positional information, and the substrates can be aligned precisely with respect to one another.
By software, the necessary calculation of the adjustment paths of the mechanical adjustment elements (alignment means) in the X- and Y-direction as well as the necessary rotation of the substrates can be calculated. This calculation and measurement can be continuously measured and corrected everywhere during the Z-movement (contacting means), i.e., the mechanical engagement of the two substrates.
Because of this online measuring process, it is possible to correct possible deviations during or after the assembly of the substrates or to ensure optimization by separating the units and providing for renewed alignment and bringing into contact.
Because of the device according to the invention and the process according to the invention, it is also possible to align precisely greatly different substrates, such as different diameters or different geometries, for example round substrates with respect to rectangular substrates.
To the extent that existing device features and/or device features in the following figure description are disclosed, the latter are also to be regarded as disclosed as process features and vice versa. To the extent that features of the substrate are disclosed in the specification, the claims or figures, the latter are also to be regarded as disclosed for the substrate-carrier substrate combination and vice versa.